1. Field of the Invention
The present invention relates broadly to the field of atomic frequency standards. In particular it relates to the class of so-called active atomic frequency standards in which coherent microwave energy is radiated by an ensemble of atoms in a cavity. In a broad sense, the system is a maser [1], [2] wherein monochromatic microwave radiation is generated by stimulated emission. More particularly the system uses the property of atoms to emit coherent microwave radiation when placed in a coherent superposition of states [3].
2. Description of the Prior Art
In some frequency standards, like the hydrogen maser [2], a beam of atoms is directed towards a storage bulb placed in a microwave cavity. Only those atoms in the upper ground state hyperfine levels F=1, m.sub.F =0 and 1 (see FIG. 1) are allowed to enter the storage bulb. This is done in the following way. First a narrow beam of atomic hydrogen is formed. The atomic beam is obtained through dissociation of molecular hydrogen in a small glass bulb by means of a radio frequency discharge at about 100 MHz. The power involved is a few watts. The atomic beam is collimated at the exit of the dissociator bulb and the atoms pass through a strong inhomogeneous-axially-symmetrical field created by a hexapole magnet. The field is zero on the beam axis and very strong at the tips of the magnet poles. Due to the properties of the atom-field interaction, those atoms in the upper energy levels F=1, m.sub.F =0 and 1 are forced towards the beam and field axis. The process results in a focusing of the atomic beam on the entrance hole of the storage bulb. They enter the said storage bulb. Only those atoms in the level F=1, m.sub.F =0 are of interest due to the fact that their energy depends on the ambient field in second order only, while the energy of the atoms in level F=1, m.sub.F =1 depends linearly on the ambient magnetic field. The storage bulb is coated internally with polytetrafluoroethylene (PTFE) or another substance such as dimethyldichlorosilane or a paraffin, that reduces considerably the relaxation of the atoms upon collision with the storage bulb inner surface. Storage times of the order of one second are possible with a bulb having a diameter of the order of 15 cm. The atoms make random transits of the bulb and interact with the electromagnetic field of the surrounding microwave cavity. When a sufficient number of atoms enter the storage bulb and when the quality factor Q of the cavity is sufficiently large, the atoms radiate their energy into the microwave cavity oscillation takes place at the ground state hyperfine frequency through stimulated emission. Atoms in the level F=1, m.sub.F =0 fall to the lower level F=0, m.sub.F =0 emitting their energy corresponding to a frequency of 1420 MHz. If the beam is maintained continuously the population of the upper level F=1, m.sub.F =0 is replenished and microwave energy is emitted on a continuous basis at a power level of the order of 10.sup.-13 W.
Continuous oscillation is possible only if hydrogen atoms enter the storage bulb at a sufficient rate and the cavity Q is sufficiently large. These criteria set a threshold on the continuous oscillation condition and put rather stringent limits on the design of the system. As a consequence the cavity design is limited to specific modes which provide a sufficiently high Q. This limits the possibility of reducing the size of the system. Furthermore, the operation of the system depends on the high efficiency of the molecular hydrogen dissociator. Maser oscillators have been operated successfully for relatively long periods and offer at present the best medium and short-term frequency stability of all the atomic frequency standards. However, the oscillation threshold has always been an important factor and has affected the size and weight of the resulting system, due mainly to the high quality factor, Q, required for the cavity. In such an oscillating maser, the cavity tuning influences in first order the frequency of emission.
The invention described in the present document bypasses the threshold conditions encountered in the prior art by preparing the atoms in a coherent superposition of energy states before they enter the storage bulb. The case of the hydrogen maser is used as an example. The hydrogen atomic beam is forced to pass in a small region where a microwave field at the hyperfine frequency of the atoms is present. When the amplitude and the frequency of the field are adjusted to the appropriate value, the atoms are put in a coherent mixed state of the two levels F=1, m.sub.F =0 and F=0, m.sub.F =0. The lifetime of this coherent state in free space is relatively very long and the atoms enter the storage bulb in that coherent state. Collision with the inner surface of the storage bulb only slightly perturb the atoms and the coherent state relaxation rate due to collisions with the PTFE or other appropriate surface coatings is of the order of one second. The hyperfine coherence present in the ground state in the storage bulb creates a magnetization in the storage bulb oscillating at the hyperfine frequency. This magnetization excites a field mode in the cavity. This field reacts back on the atoms and excites them to emit quanta of energy through stimulated emission. If the magnetization is replenished by means of a continuous beam, the ensemble emits coherent microwave energy. This emission of energy takes place without threshold in relation to the number of atoms in the storage bulb or to the quality factor, Q, of the cavity. Furthermore the cavity tuning influences the emission frequency primarily only in second order.
It is a principal object of this invention to provide a novel atomic frequency standard of the maser type wherein the atoms are prepared in a coherent state before they enter the region of emission, the microwave cavity-storage bulb assembly.
It is another object of the present invention to provide a novel atomic frequency standard of the maser type of smaller volume and weight than previously realized.
Another object of the invention is to provide an atomic frequency standard of the maser type possessing substantially improved latitude in the choice of the storage bulb shape and size.
It is still another object of the present invention to provide an atomic frequency standard having greater liberty in the choice of the microwave cavity mode, making possible, through a judicious choice of that mode or by loading the cavity with a dielectric material, a large reduction in volume and weight relative to the prior art.
It is also an object of this invention to provide an active atomic frequency standard of the maser type which emits a signal at a frequency that is dependent on the microwave cavity tuning primarily in a second order.
It is another object of the invention to provide a novel method for the generation of a coherent monochromatic microwave signal by stimulated emission in an atomic ensemble through the use of coherent state preparation.
Other objects and advantages of the present invention will, in part, be obvious and will, in part, appear hereinafter.